Symmetrize and Mirror
To know you've subdivided, you can look at your multiresolution modifier and it will say how many subdivisions you have done and will give you options for what to show for Level Viewport, Sculpt, and Render. It should also look smoother now that you've subdivided and have twice the amount of geometry.
Symmetrize does the same operation as mirror modifier, but you can turn mirror modifier off while with symmetrize you can only go back with Undo.